# No Unruh effect?

1. Nov 16, 2004

### Chronos

This is kind of like a quantum physics stand up comedy. At very least a spirited exchange of divergent ideas.

Boundary conditions in the Unruh problem
http://xxx.lanl.gov/abs/hep-th/9906181

Comment on "Boundary conditions in the Unruh problem"
http://scitation.aip.org/getabs/ser...00070000004048701000001&idtype=cvips&gifs=yes

REPLY TO 'COMMENT ON 'BOUNDARY CONDITIONS IN THE UNRUH PROBLEM'
http://www-spires.slac.stanford.edu/spires/find/hep/www?key=5966868

Spires was down when I copied these links. Should get you close. Anyways, the papers were very engaging.

2. Nov 16, 2004

### wolram

If the Unruh effect is falsified, does it follow that Hawking radiation is
also falsified ?

3. Nov 16, 2004

### Curious6

The Unruh effect says that different observers will observe different vacua, if I have understood it correctly. Does this mean also that vacua can-t really be defined anymore, because there is no "special" reference point to say what a vacua really is.

4. Nov 16, 2004

### wolram

Its disappointing that none of the big hitters want to comment on this
thread, is it not the meat and two veg of science, i for one welcome
these challenges, so any timidity can be dispelled, if anyone wants
to give a view.

5. Nov 16, 2004

### Mike2

I wonder how this observer dependent vacua relates to the "landscape" of string vacua?

6. Nov 16, 2004

### marcus

Chronos, you and I were talking about the Unruh temperature back in the thread called "Unruh: Loop Quantum Gravity paper...."

I think we agreed on a formula for the temperature. Is this it?

$$T_{BH} = \frac{1}{8 \pi M}$$

$$T_{Unruh} = \frac{a}{2 \pi}$$

these formulas are in Planck units and they say that the Bekenstein-Hawking temperature of a usual black hole horizon is proportional to one over its mass----so the more massive ones are cooler.

and it says that every acceleration has a temperature associated with it (like a "hot" car is one capable of a lot of acceleration) and
the temperature associated with some acceleration is simply proportional to the acceleration

Now for me at least I know it would be more helpful if someone could explain for me how an accelerating observer experiences this temperature. I am not going to benefit from people arguing about it, and whether the formula is right. Anyone who wants is welcome to discuss that of course but I want to hear the basic story of why Unruh got this opinion there was a temperature.

I think the key to it may be the Rindler horizon which says that as soon as you start accelerating there appears behind you a kind of barrier dividing space into two halves. There is the part where if events occur there they can causally affect you. And there is the other part, farther behind, where if events occur there they can never have any affect on you, nor can you ever observe them. because nothing from there can ever catch up

Now you Chronos know about the Rindler horizon, and how it can have a temperature just the same way as a Black Hole event horizon can have a temperature. Because a Rindler horizon is rather analogous to an event horizon.

Or maybe I am wrong. In any case it seems to me that one ought to mention whatever are the basic players in this picture and why the Unruh temp in the first place and what, for example, is a sample temperature.

Like, if i get in my rocket dragster and accelerate briefly at a steady rate of one tenth of the speed of light per second, then what Kelvins temperature do I see?

7. Nov 16, 2004

### marcus

Not to worry, wolram.
They can argue, but they are not about to disprove.

Here are the final sentences in Narozhny et al (the first paper in this exchange)

"It is worth to add that in literature the Unruh effect is usually explained by existence of event horizons for a constantly accelerated observer. But we understand that the notion of a constantly accelerated observer is an inadmissible idealization. It is clear that for any physical object the horizons are absent. We certainly understand that behavior of accelerated detectors will differ from those at rest. We admit that under some circumstances detectors of some special configuration will follow Unruh behavior. But no conclusive proof exists that this behavior is universal and does not depend on the nature of the detector and the accelerating field. "

the whole thing looks to me vague and as if it is confined purely to theory and very far from being determined experimentally one way or another.
One reason is that the unruh temperature of real accelerations is so incredibly low, like nano nano Kelvins. I am skeptical of any contemporary connection with experiment.

Despite this, I did hear one time of some experimental observation of the Unruh temperature, at Stanford SLAC, but I cant believe it. I cant picture how it could have been done
with available means. If someone knows of the experiment please give a link.

I looked at the first paper, narozhny et al.
too bad that Unruh and Steven Fulling's reply does not seem to be available online. I expect they answered back smartly enough.

This controversy may have gotten to SPR. Can anyone use the SPR search engine---at the Cornell SPR archives---and find if there is some conversation there about the Narozhny paper and whatever backlash

Last edited: Nov 16, 2004
8. Nov 16, 2004

### Kea

hi

Einstein said "In order to give physical significance to the concept
of time, processes of some kind are required which enable relations to
be established between different places."

Now this idea of a 'fixed barrier' can be improved upon with
quantum gravity, because there is no 'the other part' as you say.
We have a cosmological horizon. We have some vague idea that
if we were living at a much earlier epoch the CMBR would have
been hotter. The LOCAL measurement of the CMBR is really
our best guide to the idea that we live at a certain 'time' in
some classical sense.

9. Nov 16, 2004

### marcus

Hello, you seem to be a nice person.
I like the impression I get of your thoughts.
I am not an expert in these matters, just an observer on the sidelines watching research in various fields.

Also the CMBR does give a beautiful idea of time. One almost has the idea of a universal rest frame, and thus a universal good clock.

People all over the universe, in each galaxy, could find a local frame frame which is stationary with respect to the Hubble flow
or, so to say, NOT MOVING RELATIVE TO THE CMBR
by moving so as to eliminate the DIPOLE

and then all those observers could be approximately at one and synchronize their clocks

and they would all have the same idea of how long it has been since the "big bounce" when expansion started.

Kea, please correct me if I am wrong, but i believe this idea (which you have indicated and I am explicating) is a widespread "vernacular" idea
which many people think of.

and also there is something Carlo Rovelli once said about it----he said watch out, it does not quite work, it only works approximately, be cause
all the observers are at different levels of the gravitiational field, in different places in their galaxy and in galaxies of differenbt masses, so ALL THE CLOCKS ARE RUNNING A LITTLE UNEVENLY FROM EACH OTHER.
Or so I think rovelli said.

But this does not need to detract from the beauty of the idea of a single uniform universal time and a universal idea of rest. Well, I hope what I said is a reasonable response to you.

10. Nov 16, 2004

### marcus

but does this have much of anything to do with Unruh temperature?

11. Nov 17, 2004

### wolram

12. Nov 17, 2004

### wolram

Has the Schwinger effect been tested or proven?

13. Nov 17, 2004

### marcus

wolram you just hit a home run off a wild pitch of mine
Chen is exactly the Stanford guy I had in mind and it was, indeed, around 1999 or 2000.
However, the explanation by Matt Visser (excellent explainer!) that you found is much better than what I saw earlier and lost the link to.

So there is something LIKE the unruh temperature that might be measured, according to Mr. Chen, but Visser cautions against confusing it with unruh temperature.

I cannot answer. Actually I would guess NOT because I personally never heard of the effect being observed. But since I am not an expert, what i say is this:

1. you are really good at web-hunting. If you look for experimental observations of Schw. effect and there have been some then you will probably find them. If you dont find them then there probably arent any.
Sorry, this punts the question back to you. :)

2. maybe selfAdjoint knows, or one of the other local people who are up on particles-----arivero, vannesch, ...----so I would make a thread about it and put it in the Quantum Mecanics subforum. It is not part of quantum gravity, it is straight QFT. they should know.

BTW the Schwinger effect sounds like a fascinating thing, but the energy needed to observe it strikes me as very high. maybe we can estimate it.

14. Nov 17, 2004

### marcus

Schwinger effect is a fascinating idea.

you see near bottom of the page Visser gives a formula for the field strength that tears the QED vacuum apart.

One equivalent expression is equal to mc^2 for the electron divided by the Compton wavelength of the electron.

mc^2 is the massenergy of the electron----500,000 electronvolts.
that is how much energy is involved when an electron (or positron) goes into or out of existence.

the Compton wavelength is a very short distance that you cant pin an electron down any narrower than----it is the extent of the main hump of the matter wave (I am talking very loosely and unrigorously just to have a mental image, some kind of handle on the Compton)

the compton wavelength of the electron is 2.4 trillionths of a meter.

2.4 picometers.

Imagine a force exerted on an electron which, if it pushes that electron by only 2.4 picometers has now performed enough work to create another electron.

You have an electron sitting still in a box, with coils so you can create a field. You turn on the field suddenly.
By the time that electron has moved one compton length (its own "uncertainty size") it has already picked up enough kinetic energy from the field to create a copy of itself.

By the time it has moved one compton, it is going so fast that if it hit anything the impact could bring another electron into existence.
(to balance the books a positron would come into existence too but that is a side issue)

This is an imaginary experiment. No coils could do that. Dr. chen was saying to hit the electron with a moving wall of light from a laser. But it gives an idea of the power of the laser.

Schwinger had a nice idea. An electric field so strong that space is suddenly full of electron-positron pairs. Look mom, no vacuum. I hope you can get their attention in Quantum Mechanics forum and that they dont just make incomprehensible noises.

Last edited: Nov 17, 2004
15. Nov 17, 2004

### wolram

Task master or what, i work so hard already, but as usual thanks for
input Marcus.

16. Nov 17, 2004

### Chronos

17. Nov 17, 2004

### Kea

Hi Marcus

You seem like a nice person too.

I was not advocating a universal clock. Quite the contrary.
This is a question about real quantum observables. There
is no universal metaobserver in QG - by which I mean
the topos theoretic version.

Einstein eventually concluded that general covariance was
correct only after he had rid himself of the prejudice that
spacetime points had any physical meaning (see Stachel
on this point). Of course he had to objectify them
anyway....but only because he didn't have the maths
that we have now. In the category of sets, for
example, an element becomes a function from 'the' one
point set which selects that element. Similarly, global
sections (fields) replace points for sheaves.....anyway, even in
GR, the logic of category theory is crucial.